A GTX570 SLi has more graphics processing power than a HD6950 Crossfire. At that high of a resolution you definitely need more VRAM to cope with the extra resolution. Your GTX 570 has only 1.28GB of VRAM and not 2.56GB combined.

If you could afford a HD6950 2GB CF then you might as well go all the way for a HD6970 2GB CF. It has definitely more performance than the GTX570 and HD6950.